1. Field
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, a patterning device, which can be referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of a flat panel display (or other device). This pattern can be transferred onto all or part of the substrate (e.g., a glass plate), by imaging onto a layer of radiation-sensitive material (resist) provided on the substrate.
Instead of a circuit pattern, the patterning means can be used to generate other patterns, for example a color filter pattern or a matrix of dots. Instead of a mask, the patterning device can comprise a patterning array that comprises an array of individually controllable elements. The pattern can be changed more quickly and for less cost in such a system compared to a mask-based system.
A flat panel display substrate is typically rectangular in shape. Lithographic apparatus designed to expose a substrate of this type can provide an exposure region that covers a full width of the rectangular substrate, or which covers a portion of the width (for example half of the width). The substrate can be scanned underneath the exposure region, while the mask or reticle is synchronously scanned through the beam. In this way, the pattern is transferred to the substrate. If the exposure region covers the full width of the substrate then exposure can be completed with a single scan. If the exposure region covers, for example, half of the width of the substrate, then the substrate can be moved transversely after the first scan, and a further scan is typically performed to expose the remainder of the substrate.
Lithographic apparatus can be subject to heavy use, during which parts of the apparatus can heat up causing them to expand. This can result in errors in the illumination/exposure of the substrate. For example, if the radiation projected through the lithographic apparatus is not homogenous, the parts through which radiation is projected can expand, while the parts through which no radiation is being projected will not expand. These thermal expansion effects can be cumulative and result in positional errors in the lithographic apparatus and errors in the exposure.
During exposure of the substrate large amounts of concentrated radiation can be projected onto the substrate which will cause it to heat up and expand. The expansion of all or parts of the substrate can result in overlay errors leading to faulty devices. This problem can be particularly pronounced when large areas of the substrate need to be exposed or when very high doses of radiation are used. Additionally, the problem can be compounded by subsequent exposures, later exposures are projected onto an already partially expanded substrate.
Therefore, what is desired is a system and method that reduce errors that can result from thermal expansion of the lithographic apparatus and/or substrate.